Energetics and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3/FeAl with two different interfaces

To provide insights into the interface structure of hydrogen permeation barrier of α-Al2O3/FeAl and its effect on stability and diffusion of hydrogen isotopes, the thermodynamics and kinetics of H diffusion in α-Al2O3 (001)/FeAl (111) slab with Al/O and Al/Fe/O interfaces have been studied by the density functional theory. Hexagonal alumina layers above the FeAl plane in interface region are predicted. The interfacial binding involves cation-anion and metal-metal interactions. H-surface interaction on the α-Al2O3/FeAl slab resembles that on pure α-Al2O3 (001) slab, and the H interstitials in the α-Al2O3 part of the slab with the Al/O interface are significantly less stable than in bulk of α-Al2O3 slab, whereas that with the Al/Fe/O interface are slightly more stable. H diffusion into the α-Al2O3 part of both slabs must overcome a larger barrier of about 1.66-2.02 eV at surface-to-subsurface step, as pure α-Al2O3 case. For the bulk path, the migration of H atom can occur more readily in the α-Al2O3 part of the slab with the Al/O interface compared to that with the Al/Fe/O interface. Thus α-Al2O3/FeAl barrier with interface region of the Al, Fe mix-oxide is predicted to be much effective at protection against H permeation of the underlying steel.